Conformational Changes of Methacrylate-Based Monomers at the Air

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Conformational Changes of Methacrylate-Based Monomers at the Air-Liquid Interface Due to Bulky Substituents Uvinduni I. Premadasa, Narendra M. Adhikari, Susil Baral, Ahmed M. Aboelenen, and Katherine Leslee Asetre Cimatu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05433 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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The Journal of Physical Chemistry

Conformational Changes of Methacrylate-Based Monomers at the Air-Liquid Interface Due to Bulky Substituents Uvinduni I. Premadasa, Narendra M. Adhikari, Susil Baral, Ahmed M. Aboelenen, and Katherine Leslee Asetre Cimatu* Department of Chemistry and Biochemistry, Ohio University, 100 University Terrace, 136 Clippinger Laboratories, Athens, Ohio 45701-2979, United States

1

ACS Paragon Plus Environment


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ABSTRACT Functionalization of the monomers is important for various applications of polymers from electronics to surface coatings. Polymer chemists utilize methacrylate- based monomers to design polymers with wider application features. In this work, functionalization of the methacrylate-based monomers is preceded with varying bulky substituents, methoxy (-OCH3), ethoxy (-OEt), isopropoxy (-OiPr), tert-butoxy (-OtBu), and phenoxy (-OPh) at the ethyl end position of the methacrylate monomer. These substituents are selected to understand the steric consequences when modified to a bulkier group. This study allows determining how the substitution affects the overall conformation of the monomer at the air-liquid interface using sum frequency generation spectroscopic (SFGS) technique. The SFG spectral results were different for all monomers. To emphasize, the SFG intensity profile at ~2910 cm-1(assigned as methyl symmetric stretch,-CH3 SS) is more noticeable in the SSP spectra compared to other vibrational modes in the SSP spectra. Based on the spectral and global fitting, the change in the intensity of the ~2910 cm-1 peak was affected by the increase in the number of methyl groups in the chemical structure of the monomers. This observation has become more visible for –OiPr and -OtBu substituted monomers. Orientation distribution analysis was performed for the –CH3 SS of substituted –OCH3 and –OPh monomers using the calculated amplitude ratios of SSP and PPP polarizations. This analysis shows a narrow distribution angle of -OEt>-OiPr >-OtBu substituents. Then the –OPh substituted monomer, 2 –phenoxyethyl methacrylate (PhEMA), had a high ST value which is the result of the intermolecular forces including pi-stacking interactions. The ST value of PhEMA favored the SFG spectral data because the aromatic CH stretch was observed and can only be detected if the –OPh group is oriented perpendicular to the surface plane. These results facilitate understanding the effects of the substituents on the conformations of monomers. Moreover, these results will help correlate these effects with the physicochemical properties of the respective polymers. Introduction Polymers have a wide variety of applications with their properties especially dependent on the nature of their chemical structures. A common method in developing polymers is through varying their structures. These modifications of polymers via functionalization have been studied to understand and improve their physicochemical properties.1-4 Moreover, understanding the properties of these polymers can then be associated with the structure of their monomers which renders monomers as a significant component in creating better polymers. For example, methacrylate- and acrylate-based monomers have been widely modified and used for different applications in the industry. Because methacrylate-based monomers (MBMs) can be readily polymerized by means of radical polymerization.5-8 These methacrylate-based monomers are used by polymer chemists in the industry to design polymers with a broad range of valuable characteristics. These characteristics include durability, stability, resistance to corrosion, etc.9-11 With these characteristics, MBMs are used for various applications in electronics and computers12, paints and surface coatings13-16, biosensors17, dentistry,18-21 adhesives,22-23 and 3



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porous materials for chromatography.24 These methacrylates also have antibacterial, antifungal, toxicological, and pharmacological properties.25-28 Therefore, these methacrylates are found useful in biomedical applications, especially for tissue engineering and medical devices.29-30 Though methacrylate-based monomers are used for designing polymers with different properties, it is common to characterize the polymers instead of the monomers to understand the effects of any modifications to the bulk or surfaces of the polymers. Studying the monomers more methodically is required to improve the features of polymers because these monomers can be arranged in different configurations. These features can then be associated with the types of polymerization the polymers undergo such as addition and condensation. These two processes can form linear, branched, or cross-linked polymers. These configurations of polymers in effect affect their properties. One main chain of molecules with smaller chains appended to it is socalled branched polymers. The different arrangements of molecules affect the polymer by lowering its degree of crystallinity and density.31 Thus studying the structures and conformations of functionalized monomers at the air-liquid interface can provide a detailed approach to understand the resulting configurations. Previously, we have characterized the hydroxy-, chloroand phenoxy-substituted monomers and their homopolymers at the air-liquid interface and found that the alkene-methylene C-H vibrational mode (~3000 cm-1) in monomers was not observed in polymers.32 Moreover, the 2-methoxyethyl methacrylate (MEMA) monomer was found to be partially ordered at the air-liquid interface after orientational analysis using polarization combination and polarization mapping techniques.33 In this project, functionalized methacrylatebased monomers are selected because of their availability and straightforward synthesis by modifying their chemical structures through acyl substitution. The project focuses on 1) understanding the steric consequences of changing the substituent attached to the ethyl end 4


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position of the methacrylate monomers to a bulkier group, 2) synthesizing of isopropoxy and tert-butoxy substituted methacrylate-based monomers and then 3) identifying how the substitution affects the overall conformation of the monomer at the air-liquid interface using sum frequency generation spectroscopic (SFGS) technique. SFGS is a valuable technique for determining the interfacial molecular conformation. In case a bulk analysis is performed, the steric influence from the substituted group may have a small effect on the change in the overall molecular conformation. On the other hand, the substitution can have a discernable effect on the conformation at the air-monomer interface. As the steric influence is increased by introducing the bulky substituents, it can be hypothesized that these bulky substituents will affect the spectral profile of the SFGS spectra resulting to a change in the overall molecular conformation. The investigation of how bulky groups affect the conformation of the monomers is important because previous studies reported that the optical properties, reactivity, thermal stability, electrochemical properties, polymerization process and polymer properties are directly influenced by the chemical structure of the monomers and the steric hindrance the monomers experience.34-39 The substitution of –methoxy(-OCH3), -ethoxy (-OEt), -isopropoxy (-OiPr), -tertbutoxy (-OtBu), and phenoxy (-OPh) of the 2-substituentethyl methacrylate monomers can provide a systematic understanding of how the addition, for example, of –CH3 groups changes and affects the overall interfacial conformation using SFGS. Sum frequency generation spectroscopy (SFGS) is a suitable tool to probe the molecular groups at the interface where the symmetry is broken.40-41 SFGS is a second-order nonlinear spectroscopic technique that can be used to investigate different light-accessible interfaces. This surface sensitive technique has been extensively used to study the interfacial conformation of polymers in various chemical environments to understand their properties.42-44 One study, in 5



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particular, had reported a modification of the chemical structure and how the variation affected the conformation of the polymer. Zhan Chen and coworkers performed this experiment with increasing alkyl substituents at the ester side chain, and it revealed that for poly (n-alkyl methacrylate)s (PAMAs), the variation of the chain length affects the orientation of the –CH3 end group. As the chain length was increased, the relative intensity of the methylene symmetric stretch has dominated the SFG spectrum. This indicated that more and more gauche defects were detected at the air-polymer interface which in effect, also affected the orientation of the terminal methyl group of the side chain. Moreover, surface tension (ST) of the PAMAs has also been evaluated.45 SFGS has also been used to study monomers such as ethylene glycol 46 including our previously published papers.32-33 The basic theory of SFGS is available and reported elsewhere.32-33, 40, 47-53 This technique generates vibrational spectra of molecules that are present at the interface, which is in a noncentrosymmetric environment. SFG spectra can be analyzed to provide information about the orientation and molecular conformation using different polarization combinations.

51, 53-54

Also,

the SFG signal depends on the incident angles, orientation and number density of the vibrational modes at the interface. The position, intensity and the phases of vibrational resonances of SFG spectra enable us to obtain information such as presence, orientation, and the coverage of interfacial molecules. 41, 55

The second order nonlinear susceptibility (𝜒 (2) )is an important parameter in SFGS as it is

solely responsible for the vibrational information obtained from the SFG spectrum. 𝜒 (2) is (2)

defined as the macroscopic average of molecular hyperpolarizibilities (β), and denoted as 𝜒𝑖𝑗𝑘 in laboratory coordinates system (i, j, k). In the Cartesian coordinates system (x, y, z), this third 6


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rank tensor has 27 different components and due to symmetry considerations, an isotropic (2)

(2)

(2)

(2)

(2)

(2)

(2)

medium will contain only 7 non-zero components. 𝜒𝑥𝑥𝑧 , 𝜒𝑦𝑦𝑧 , 𝜒𝑧𝑧𝑧 , 𝜒𝑥𝑧𝑥 , 𝜒𝑧𝑥𝑥 , 𝜒𝑧𝑦𝑦 and 𝜒𝑦𝑧𝑦 are selected in a way, that y is along the surface normal and xz is along the incident plane. Different tensor components can be deduced through measuring SFG spectra using different polarization combinations of the incident beams and the generated SFG beam. S- polarization will have y component and P- polarization will contain both x and z components. These polarization combinations enable us to determine the orientation of the molecular groups at the interface.33, 41, 55-56

The detailed theory is reported elsewhere. 32-33

On the other hand, 𝜒 (2) is also related to the SFG intensity as shown by Equation 1. 𝜒 (2) consists (2)

(2)

(2)

of 𝜒𝑅 and 𝜒𝑁𝑅 , the resonant and non-resonant components, respectively. 𝜒𝑅 is a term related to 𝛽, hyperpolarizability, the product of IR dipole moment and Raman polarizability tensor.55, 5758

𝐼𝑆𝐹𝐺 ∝ |𝜒 (2) |2 ∝ |𝜔

𝑁 𝑞 −𝜔𝐼𝑅 +𝑖𝛤𝑞

(2)

2

+ |𝜒𝑁𝑅 |𝑒 𝑖𝜌 |

(1)

where N is the number density of vibrational transitions and 𝛤𝑞 is the damping constant of the qth vibrational mode. 𝜔𝑞 and 𝜔𝐼𝑅 are the resonance and the incident IR frequencies, respectively. 𝜌 (2)

is the phase of the non-resonant response. Generally, 𝜒𝑁𝑅 is considered to be negligible for liquids and dielectric surfaces.32,

57-58

However, in this study, the non-resonant contribution is

also considered in the fitting equation, in order to account for any contribution from the bulk. The simplified version of the fitting equation is shown below. 𝐼𝑆𝐹𝐺 (𝜔 + 𝜔𝑣𝑖𝑠 ) ∝ 𝑒𝑥𝑝 [−

𝐿 2 (𝜔−𝜔𝐼𝑅 )

2(𝛿𝜔𝐿 )2

] × |∑𝑞 𝜔

𝐴𝑞 𝐼𝑅 − 𝜔𝑞 +𝑖Γ𝑞

2

+ 𝐴𝑁𝑅 𝑒 𝑖𝜌 |

(2) 7



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The contribution from the broadband width of the IR beam profile is considered by including the 𝐿 33, 59 Gaussian function with the spectral width of 𝛿𝜔𝐿 centered at 𝜔𝐼𝑅 . The amplitude factors, 𝐴𝑞

and 𝐴𝑁𝑅 , are proportional to 𝛽 as shown in Equation 1. 59-60 By performing experiments with different polarization combinations, the relationship between different 𝜒 (2) can determine the orientation of the bond. For an isotropic liquid surface, these polarization combinations can generate SFG signal. Each of the polarization combinations probe different components of 𝜒 (2) . In this study, functionalized methacrylate-based monomers are 1) modified with substituents with increasing bulkiness 2) investigated at the air-liquid interface using SFGS and 3) characterized using a tensiometer for surface tension measurements. The backbone structure of the five selected monomers are identical, as shown in Figure 1, and the substituents are varied accordingly from -OCH3 to -OEt ,-OiPr, -OtBu, and -OPh giving 2methoxyethyl methacrylate (MEMA- nCH3= 1), 2-ethoxyethyl methacrylate (EEMA- nCH3= 2), 2isopropoxyethyl methacrylate (IEMA- nCH3= 3), 2-tertbutoxyethyl methacrylate (TEMA- nCH3= 4), and 2- phenoxyethyl methacrylate (PhEMA- nCH3= 1), respectively. The total number of nCH3 includes the α-methyl group of the methacrylate backbone. Besides SFGS, infrared and Raman spectroscopic techniques are used to ensure proper peak assignments of the vibrational modes arising from the molecular groups. MEMA, EEMA, and PhEMA monomers are commercially available. Whereas IEMA and TEMA monomers are synthesized and fully characterized to ensure their chemical structures and purities. In addition, methyl methacrylate (MMA) monomer is characterized to reduce peak interference from methylene (CH2) groups and support the vibrational peak assignments of the observed peaks in the SFG spectra of MEMA, EEMA, IEMA, TEMA, and PhEMA. The SFG spectra are acquired for MEMA, EEMA, IEMA, TEMA, 8


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and PhEMA monomers. Then, the spectra are fitted for peak assignment and used to determine peak amplitudes. The orientation distribution analysis was performed for both MEMA and PhEMA because both monomers have distinct alpha-methyl (α-CH3) group in their structures. The amplitude ratios of methyl symmetric stretch (-CH3 SS) in SSP and PPP for MEMA and PhEMA are used in estimating the orientation distribution of the -CH3 SS from the α-CH3 group. The SFG results are also related to the surface tension measurements.

A

B

C

D

E

F

Figure 1. Chemical structure of the functionalized methacrylate-based monomer as A)2-methoxyethyl methacrylate (MEMA, -OCH3, nCH3 = 1), B) 2-ethoxyethyl methacrylate (EEMA, -OEt, nCH3=2), C) 2-isopropoxyethyl methacrylate (IEMA, -OiPr, nCH3=3), D) 2-tertbutoxyethyl methacrylate (TEMA, -OtBu, nCH3=4), E) 2-phenoxyethyl methacrylate (PhEMA, -OPh, nCH3= 1), and F) methyl methacrylate (MMA).

Experimental Methods Materials and Methods Eicosanoic acid (EA) (≥ 99.0%) was purchased from Sigma-Aldrich and used as received. Commercially available 2-methoxyethyl methacrylate (MEMA) (99%), 2-ethoxyethyl methacrylate (EEMA) (99%, containing 100-ppm hydroquinone (HQ) and 200-ppm monomethyl ether hydroquinone (MEHQ), and methyl methacrylate (99%, containing 2900 cm-1 are less convoluted because of the CH3 AS (α-CH3 group) positioned at 2963 cm-1 is visible in the SSP spectrum of MMA monomer. The better peak resolution is due to the lack of interference from the –CH2 groups. To emphasize, the α–CH3 AS is not visible in the SSP spectrum of other monomers because of overlapping vibrational modes. Also, the peak at ~2891 cm-1 is still present in the SSP spectrum of MMA which verifies that this vibrational mode arises from the α-CH3 group. Table 1. Vibrational assignments of the peaks of the five monomers in the IR, Raman, SFG spectra in SSP and PPP polarizations. General Vibrational Peak Assignments of the Monomers

Wavenumber Range/ cm-1 IR

Raman

2732-2738

2732-2743

2820

2826

2842-2876

2846-2877

2861-2872 2861-2872

4) CH3 symmetric stretch 71-72, 75-76 5) CH3 symmetric Stretch- FR (SSP) 76-78/ CH2 asymmetric stretch (PPP) 76, 79 6) CH3 asymmetric Stretch (inplane) 55, 80 7) CH3 asymmetric Stretch (out-ofplane)55, 81 / -OCH3 asymmetric stretch (MEMA) 55, 71, 76, 82-84

2879-2908

2885-2899

2902-2912 2902-2912

2928-2930

2930-2931

2934-2940 2934-2940

2954-2974

2957-2980

2959-2970

2979-2983

2980-2994

2980-2991

8) Alkene- methylene CH stretch85

3014-3021

2999-3028

3005-3017

9) Aromatic CH stretch 86 10) Presence of hydroxyl peaks of adsorbed water 83, 87-89

3043,3064

3043, 3071

3053

3103-3190

3108-3172

65

1) Unassigned 2) Methoxy (-OCH3) symmetric stretch 66-69 3) CH2 symmetric stretch (SSP) 66, 69-72 / Fermi related methylene group vibrational mode (PPP)7374

SSP

PPP

2737-2751 2737-2751 2819

2819

3053

3189-3151 3189-3151 19



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SFG spectral profile variation of the functionalized methacrylate-based monomers The differences in the spectral profiles of the functionalized methacrylate-based monomers are presented in this section, which is the direct consequence of the substitution of bulky groups. Comparison of the presence or minimal visibility of vibrational modes in the SFG spectra and variations in the intensity profiles are also discussed. First, the molecular group of – OCH3 was bound to the ethyl end of the methacrylate monomer (check Figure 1 for the chemical structure). The normalized SSP and PPP SFG spectra of the MEMA monomer are different from EEMA, IEMA, TEMA, and PhEMA because of the prominent peak of the –OCH3 SS observed in the SSP spectrum. The vibrational mode of the –CH3 group has shifted to a lower frequency when bound to an oxygen atom. The shifting from ~2875 cm-1(for a typical methyl group –CH3 symmetric stretch vibrational mode peak assignment)90-91 to ~2819 cm-1 is due to influence of the oxygen atom connected to the –CH3 group.92 The second broad convoluted peak in the PPP spectrum is an overlap of the –CH3 AS (from the α-CH3 group) and –OCH3 asymmetric stretch (OCH3 AS, from the –OCH3 group). This –OCH3 AS peak positioned at ~2985 cm-1 could also be coming from the out-of-plane CH3 asymmetric stretch of the α-CH3 group. We do not have a method of differentiating the two vibrational modes. For now, we can only infer that the second peak could be from the two vibrational modes. The next substitution was the replacement of the ethyl end with –OEt (–OCH2CH3) in the methacrylate-based monomers. To note, the SSP spectrum is similar to MEMA except for the absence of the –OCH3 SS vibrational mode. There are three noticeable peaks positioned at ~2868 cm-1, ~2903 cm-1, and ~2939 cm-1. The CH stretch (~3016 cm-1) from the alkene-CH2 group is not as noticeable as the other three vibrational modes. Another distinguishable difference is the variation in the PPP spectra. In MEMA, there were two broad peaks, of similar intensity but in 20


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EEMA, the first broad peak was less noticeable compared to the second broad peak. The assignment of the first broad peak in MEMA is a convolution of the –OCH3 SS and Fermi related –CH2 SS. The two aforementioned vibrational modes are from –OCH3 and –CH2 groups. Since the –OCH3 group has been replaced in EEMA, all vibrational modes from this group disappear. On the other hand, the SFG signal from the –CH2 group will not be entirely diminished, but instead, the signal is minimized because we still have three methylene groups in the monomer. This Fermi related –CH2 vibrational mode can be observed as a shoulder beside the ~2868 cm-1 peak. As we add one more –CH3 group through the attachment of –OiPr (-OCH(CH3)2) molecular group (IEMA) at the ethyl end of the methacrylate-based monomer, the peak positions of the SFG spectra are the same except that the intensity profiles are different for both SSP and PPP polarization combinations. The difference in the intensity profiles, especially the observable intensity change of the peak positioned at ~2900 cm-1 can be due to either 1) number density of methyl group and/or 2) the relative orientation and orientational order of the methyl group (from the α-methyl and the –CH3 groups at the ethyl end of the functionalized monomer) at the airliquid interface. At this point, it is difficult to determine the specific source of the –CH3 SS and – CH3 AS modes to cause a change in the intensity profile except for a fact that the structure of the monomer’s backbone is the same, and the ethyl end is the part of the monomer that is varied. In the future experiments, the deuteration or replacement of the α-CH3 group, or use an alternative monomer such as an acrylate group (without the α-CH3 group) are being considered to help figure which molecular group is the source of the –CH3 SS. The next substitution is attaching the –OtBu group (TEMA) at the ethyl end of the monomer. The spectral profile is similar to IEMA except that the intensity profile is different. 21



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The same interpretation can be applied to describe the intensity profile changes in the SSP and PPP spectra of the TEMA monomer. The intensity profile variation can be due to the number density of the –CH3 SS vibrational mode and/or the overall orientation of the –CH3 group. Lastly, we attached a –OPh group (PhEMA) to the methacrylate-based monomer. The spectral profile of the PhEMA monomer is different from the first four functionalized monomers. The SSP spectrum contains vibrational modes from the –CH2, α-CH3, alkene-CH2 and the –OPh groups. The rich spectrum of the PhEMA monomer is notable as it contains peaks from the molecular groups present in its chemical structure except for the >C=O and C=C group at the airliquid interface. Compared to EEMA, IEMA, and TEMA, the alkene-CH2 CH stretch positioned at ~3005 cm-1 is still noticeable in the SSP spectrum of PhEMA. Thus, the =CH2 group is present at the air-liquid interface. Moreover, the aromatic CH stretch from the –OPh group is also visible in the SSP spectrum. The presence of the aromatic CH stretch in both the SSP and PPP spectra can be due to the –OPh substituent favorable arrangement at the air-liquid interface. A change in the intensity profile of a spectrum for a certain vibrational mode can either be due to the change in the number density or orientation at the air-liquid interface. The number of –CH3 groups increases from the substitution of –OEt (nCH3 = 2), -OiPr (nCH3 = 3), and –OtBu (nCH3 = 4) to the ethyl end of the methacrylate-based monomer. The total number of methyl groups includes the α-CH3 group. Moreover, the number density is difficult to verify. The determination of the orientation distribution of the –CH3 SS to evaluate how this substitution of the bulky groups (with increasing number of –CH3 groups) affects the orientation of the methyl groups at the air-liquid interface cannot also be validated for EEMA, IEMA, and TEMA monomers. The orientation distribution analysis of the CH3 SS for EEMA, IEMA and TEMA is not determined because the source of the CH3 SS vibrational mode is difficult to determine 22


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whether from an alpha methyl group or the methyl groups of the substituents. On the other hand, the distribution analysis can still be performed for MEMA and PhEMA monomers because they both only have the α-CH3 group as the sole source of the –CH3 vibrational modes. Therefore, the amplitude ratio of the -CH3 SS in SSP to PPP polarization combination was calculated from the fitting results of both monomers. Then, the ratios were compared to the simulated curves to obtain the orientation distribution of the –CH3 symmetric stretch. With the discussions presented above, the normalized SSP and PPP spectra of MEMA, EEMA, IEMA, TEMA, and PhEMA are globally fitted using Equation 2. The global fittings are to help obtain peak positions and the SFG amplitudes of the –CH3 SS positioned at ~2900 cm-1. As noted, the spectra were normalized to the eicosanoic acid which is used as a reference. The fitted SSP and PPP spectra of all five monomers are provided in Figure 3. The fitting results of these five monomers are provided in Tables S7-S11 in the SI.

0.18 0.16 5

0.14 0.12

4

0.10 0.08 3 2

0.06 0.04 0.02 0.00 2600

1

2700

2800

2900

3000

3100

3200

Normalized SFG Intensity (a.u.)

0.20

Normalized SFG Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.20 0.18 0.16

5

0.14 0.12 0.10 0.08

4

0.06 3

0.04

2

0.02 0.00 2600

1

2700

Wavenumber (cm-1)

2800

2900

3000

3100

3200

Wavenumber (cm-1)

A

B

Figure 3. A) SSP and B) PPP SFG spectra of MEMA (black dot and line), EEMA (blue dot and line), IEMA (green dot and line), TEMA (purple dot and line), and PhEMA (brown dot and line) monomers.

23



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Distribution Analysis of the α-CH3 Symmetric Stretch in MEMA and PhEMA Monomers The amplitude ratios of the –CH3 SS between SSP and PPP polarization combination were calculated from the global fitting results and then compared to the simulated curve generated for the orientation distribution. The detailed SFG theory including the generation of the simulated curve has been previously reported by our group.33 The parameters used to generate the simulated curve are reported in Table S12. The same refractive indices were used for all the monomers at the air-liquid interface (Table S12). Estimating the distribution of the tilt angle is more convenient when studying the orientation of the interfacial molecules especially when these groups are associated with multiple or different tilt angles. The distribution of the tilt angle is a good indication of the surface coverage for a specific vibrational mode. The simulated curve for the amplitude ratio of the -CH3 SS between SSP and PPP polarization was plotted as a function of the distribution of the tilt angles. The distribution of the tilt angles follows a Gaussian distribution (GD) and equation 3 is used for generating the simulated curve.

𝐺𝐷 =

1 𝜎𝑑𝑒𝑔√2𝜋

𝑒

−

(𝜃−𝜃0)2 2𝜎2

(3)

where 𝜃 is the tilt angle, 𝜃0 is the average tilt angle, 𝜎 is the standard deviation and 𝜎 2 is the variance which shows the deviation about 𝜃𝑜 . The angles are in units of degree. 𝜎𝑑𝑒𝑔 is the distribution angle for the average tilt angles. The simulated SFG amplitude ratio is plotted as a function of 𝜎𝑑𝑒𝑔 . The following equation is used to express the amplitude distribution (AD). 180

𝐴𝐷 = 𝐴𝑏𝑠 [

∫0

(𝑛𝜒𝑒𝑓𝑓 sin[𝜃]∗𝐺𝑎𝑢𝑠𝑠𝑖𝑎𝑛 𝑑𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛)𝑑𝜃 180

∫0

(sin[𝜃]∗𝐺𝑎𝑢𝑠𝑠𝑖𝑎𝑛 𝑑𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛)𝑑𝜃

]

(4)

24


Page 25 of 46

where 𝜒𝑒𝑓𝑓 is the representative of the polarization combination. In our case, we used SSP and PPP polarization combinations. The effective susceptibility,𝜒𝑒𝑓𝑓 was obtained for the –CH3 SS from a C3v point group. Equation 5 was used to obtain the simulated normalized amplitude ratio distribution (ARD) of the simulated curve plotted versus the distribution angle: 𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝐷𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶3𝑉𝑠𝑠𝑆𝑆𝑃

𝑁𝑜𝑟𝑚𝑎𝑙𝑖𝑧𝑒𝑑 𝐴𝑅𝐷 = 𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝐷𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶3𝑉𝑠𝑠𝑃𝑃𝑃

0

7 6 5 4 3

8.0

o

10

o

20

o

30

o

40

o

50

o

60

o

70

o

80

o

90

o

Simulated SFG Amplitude Ratio

8


(5)

Amplitude Ratio = 7.6 +/- 16

7.8

60

o

70

o

80

o

90

o

MEMA

7.6

O

0.2

O

15.5

O

2.4

O

18.7

O

19.6

7.4 7.2

MEMA PhEMA

2

7.0

1 0

20

40

60

0

80

2

4

6

Distribution Angle,

8 10 12 14 16 18 20 22 24


A

B 2.5 Amplitude Ratio = 1.9 +/- 1.7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

o

10

o

20

o

30

o

PhEMA

2.0

O

13.0

O

18.8

O

22.8

1.5 5

10

15

20

25

30

35


C Figure 4. Simulated curves of the SFG amplitude ratios between the SSP and PPP as a function of the distribution angle are shown for A) two monomers MEMA and PhEMA, B) MEMA, and C) PhEMA. A) is shown without the error bar for clarity purposes. The curve with the error bar is available in Figure S11 of the supporting information. B) and C) are presented in a zoomed scale for a clearer view. As indicated, there are several possible distribution angles for several specific tilt angles. The horizontal solid lines indicate the calculated amplitude ratio for CH 3 SS (SSP/PPP) from the fitting of the spectra.

25



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Page 26 of 46

Using the simulated curve, Figure 4A shows the distribution angle of the orientation for CH3 SS vibrational mode of MEMA and PhEMA monomers. The color-coded dashed lines are a representation of the amplitude ratios calculated for the -CH3 SS (SSP/PPP) of MEMA and PhEMA monomers. The calculated amplitude ratios of the two monomers from fitting the SFG spectra are listed in Table 2 together with the orientation distribution. Overall, the two monomers show a relatively narrow distribution at < 30° for the multiple tilt angles listed in Table 2. Figures 4B and 4C are representations of the orientation distribution of the two monomers separately. The figures are shown with larger representations of the graphs by zooming in for a clearer view of the distribution angle for the average tilt angles of each monomer. For example, at a tilt angle of 70°, the distribution angle for the MEMA monomer is estimated to be 15.5° from an amplitude ratio of 7.6 ± 16. From the result, the distribution angle obtained is narrow at a tilt angle of 70° for the MEMA monomer. Since the distribution angle is narrow, most of the –CH3 SS vibrational mode is oriented at a tilt angle of 70° closer to the surface. This can also be interpreted that the narrow distribution means a small deviation from a mean tilt angle. Moreover, the monomer seems to adapt to a more horizontal orientation. The PhEMA monomer, conversely, has its calculated amplitude ratio of 1.9± 2 obtain relatively narrow distribution angles of 0°, 10°, and 20° average tilt angles. The α-CH3 symmetric stretch of the PhEMA monomer is oriented from 0° - 20°, but with a narrow distribution at the air-liquid interface. The orientation distributions with errors calculated at 95% confidence level (CL) are summarized in Table 2. The high uncertainty values for the amplitude ratios can be accounted for the result of fitting convoluted spectra. We minimized the errors in the estimation of the orientation distribution by globally fitting the SFG spectra. 26


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Table 2. Orientation distribution analysis of the methyl symmetric stretch (-CH3 SS) at the air-liquid interface using the amplitude ratios between SSP and PPP polarization combinations for the CH 3 SS. Uncertainty error was calculated with a 95% confidence level.

Monomers

Amplitude Ratio of CH3 SS (SSP/PPP)

MEMA

7.6± 16

PhEMA

1.9± 2

Orientation Distribution 2.4°± 40° 15.5°± 32° 18.7°± 39° 0.2°± 0.4° 19.6°± 41° 22.8°±20° 18.8°± 16° 13.0°±12°

(60° tilt angle) (70° tilt angle) (80° tilt angle) (90° tilt angle) (90° tilt angle) (0° tilt angle) (10° tilt angle) (20° tilt angle)

This orientation distribution analysis conveys an idea about the orientation distribution of a functional group at an average tilt angle. The 𝜎deg value denotes whether an average tilt angle is narrowly or broadly distributed. As mentioned above, the orientation distribution results of MEMA, the multiple average tilt angles of 60°, 70°, 80°, and 90° are narrowly distributed with distribution angles MEMA> EEMA >IEMA > TEMA (Figure 6). The functionalized methacrylate-based monomers have a small complex structure as shown in Figure 1 with–CH3, -CH2, =CH2 groups, etc. The SFGS results provided the SFG spectra that are dominated by vibrational modes from –CH3, -CH2, =CH2 groups, etc. at the air-liquid interface. The presence of these organic moieties at the interface is then further verified by the surface tension values obtained for the monomers. For instance, this trend in the surface tension values has also been studied and observed for alkyl sulfates ionic liquids with increasing chain length or a number of carbon atoms (methyl, ethyl, propyl, and butyl).94 Moreover, using Monte Carlo simulations, the surface tension value is predicted to be higher for phenol compared to primary and secondary alcohols and decreases among alcohols with increasing alkyl chain or branching.99 The comparison of the surface tension values of alkyl sulfates showed the effect of increasing chain length or number of carbon atoms to the surface tension values. Other reports have also shown that the determination of 32


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critical surface tension of spreading on a liquid substrate is affected by the inherent molecular structures of saturated and unsaturated hydrocarbons (HCs) and the intermolecular interactions that exist between these molecules. In these studies, the surface tension values obtained for increasing chain length with the same number of –CH3 groups resulted in decreasing ST values, increasing branching resulted in decreasing ST values and lastly, as the aromaticity increased, the ST values increased. The possible reason for the increased ST value of aromatic hydrocarbons could be the sum and coexistence of intermolecular forces at the surface other than the dispersion forces.100-101 This observation with decreasing value of surface tension as the chain length or number of carbon atoms increases can be correlated with our results wherein the MEMA (30.5 mNm-1), EEMA (26.9 mNm-1) with the addition of one –CH3 group, IEMA (24.9 mNm-1) with two –CH3 groups, and TEMA (23.8 mNm-1) with three –CH3 groups follow the same trend, as shown in Figure 6. In our case, as we increase the number of –CH3 groups, the measured surface tension values also decreased. The decreasing surface tension trend from MEMA to TEMA can be explained in terms of increasing number of –CH3 groups at the ethyl side of the monomers and low polarity of these monomers.100 This is consistent with hydrophobic nature of the molecules and increasing attraction to air, which suggests an increase in the total of the dispersive forces. In relation to the SFG spectral results, an increase in the signal intensity was observed for the –CH3 symmetric stretch vibrational mode (peak positioned at 2910 cm-1) complements the overall trend of the surface tension measurements. Overall, the addition of bulky substituents at ethyl end of the functionalized methacrylate-based monomers increased the hydrophobic nature of the monomers; decreasing surface free energy.

33



38

Surface Tension(dynes/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 46

36 34 32 30 28 26 24 22 PhEMA MEMA

EEMA

IEMA

TEMA

Methacrylate monomers Figure 6. Correlation of the ST values with the monomers. The error bars are estimated for the 95% confidence level.

Moreover, the measured surface tension value of PhEMA monomer is higher at 36.9 mNm-1 compared to the four monomers. This measurement agrees with another report which reported that the surface tension value for phenol is higher compared to methanol, ethanol, or propanol.99 This increase in the ST tension value of the PhEMA monomer is a result of the increase in intermolecular interactions. This attractive intermolecular forces can be due to pistacking100 besides the already existing dispersion forces of the aromatic (phenyl) ring structure. Overall, the phenyl ring attached to the molecular structure has an increasing effect on the surface tension because of the electron cloud distortion and its ability to stack together.46 The relationship between the ST measurement and the SFG spectral results of the PhEMA monomer can be associated with the presence of the –CH2, α-CH3, =CH2 groups in addition to phenoxy group (aromatic CH stretch) at the air-liquid interface. These observations of these groups are a good indication that these groups are contributing to the interface. Since the characterization was performed at the air-monomer interface with air having a hydrophobic nature, the –OPh groups 34


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could position themselves at the air-liquid interface.

Its planar structure also allows the

arrangement of these phenyl rings adjacent to the other phenyl rings.62 In addition, the measured ST value of the PhEMA value is still lower than the measured ST value of water (75.1 ± 0.1). Thus, the PhEMA monomer is still considered to be dominated by its hydrophobic nature. A representation of the PhEMA monomer is presented in Figure 7, as to how we visualize the monomer at the air-liquid interface. Lastly, the effect of temperature should be considered in the interpretation of the SFG spectral results with surface tension data because surface properties are also dependent on any temperature changes. For example, when the temperature increases, there will be an increase in the molecular thermal activity. The change in the molecular thermal activity then affects the cohesive forces to weaken which results in a decrease in the surface tension of liquid. In effect, any changes in the temperature also influence the adhesive forces experienced by the liquid at the interface. Moreover, studies have reported the effect of temperature on the surface tension of organic liquids. As stated, the surface tension of the liquid linearly decreases as a function of temperature.102-103 In addition, it was also reported that the change in surface tension value of a liquid is negligible over a range of small temperature differences.104 In this study, the SFG experiments and surface tension measurements were carried out at 24oC and 20oC, respectively. Based on the reported studies, the 4oC difference between the SFG and surface tension measurements should not affect the interpretation of the acquired SFG spectra and the measured ST values of the monomers because the surface tension is a linear function of temperature. This means that even if the value of the surface tension changes as a function of the temperature, the measured ST values change with temperature linearly. Thus, the interpretation of the trend in relation to the SFG results is not affected by a 4oC temperature difference and vice versa. 35



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Page 36 of 46

Figure 7. A representation of the how PhEMA monomers with –OPh substituent organize at the air-liquid interface.

Conclusion Functionalized methacrylate-based monomers with bulky substituents affect the overall conformations of the molecular groups at the surface, as observed using sum frequency generation spectroscopy. The monomers selected in this study had the substituents –OCH3, -OEt, -OiPr, -OtBu, and –OPh at the ethyl end of the methacrylate-based monomers. IEMA and TEMA monomers were synthesized in-house to complete the series. The monomers were characterized using SFGS at the air-liquid interface. The functionalized methacrylate-based monomers were compared with each other to their spectral results. Then, the surface tension measurements were performed and correlated with the SFGS observations. It is also good to note that the –OCH3 symmetric stretch of the MEMA monomer was observed at ~2819 cm-1 in SSP polarization. Whereas, in its PPP polarization, two broad peaks were observed and assigned to combinations of vibrational modes. EEMA’s spectral profile is similar to MEMA except for the SSP spectrum does not contain the –OCH3 symmetric stretch peak. Also, the PPP spectrum only has one noticeable broad peak assigned to the combination of –CH3 asymmetric stretch and the out-ofplane –CH3 asymmetric stretch. EEMA, IEMA, and TEMA monomers spectral profiles are similar, but the intensity profiles are different. On the other hand, the SSP and PPP spectra of the 36


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PhEMA monomer are different from the other monomers because of the observation of the peak assigned as the CH aromatic stretch. Thus, the overall conformation was affected by the bulky substituents of the methacrylate-based monomers. The change in the SFG intensity profile/ amplitudes identified more noticeably with the peak positioned at ~2910 cm-1 which can be accounted to the number density or orientation distribution. Orientation distribution analysis was performed for MEMA and PhEMA monomers to account for possible multiple tilt angles. The distribution angles obtained for the –CH3 symmetric stretch of the MEMA monomers are

Conformational Changes of Methacrylate-Based Monomers at the Air

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